Directional couplers are widely used in microwave and RF circuits as separate components, or as parts of other devices. They are used separately for power dividing/combining, for power monitoring and isolation of dc components. They are parts of the following devices: directional filters, mixers, phase shifters, attenuators, balanced amplifiers, magic-tees, modulators, beam-forming networks for array antennas, etc.
Directional couplers can utilize different waveguiding media, for example waveguides, coaxial lines, printed transmission lines—like microstrip, strip-lines, coplanar lines, etc. Printed directional couplers use pieces of single or coupled lines placed on, or between, planar dielectric substrates. Directional couplers made of coupled lines have wider frequency bandwidth.
There are many of known configurations of coupled-line directional couplers. The typical structure can utilize coplanar-coupled or broad-edge-coupled microstrip or strip-line transmission structures. Prior art microstrip and coplanar structures, cross sections of which are shown in FIGS. 1(a), (b) and (c), utilize paired parallel transmission lines in the same horizontal plane. They function predominantly as inductive coupling structures, which means that the inductive coupling coefficient is greater than the capacitive one. As seen in FIG. 2, the broad edge-coupled structure positions the coupled transmission lines such that the second line overlaps the first one along the vertical axis. The broad-edge topology functions predominantly as a capacitive coupling structure. In this case the capacitive coupling coefficient is greater than the inductive one. If coupling coefficients are different, a coupler is ‘not compensated’, and has poor directivity. Among the many techniques that can be used to equilize the inductive and capacitive coupling coefficients (to compensate a coupler) include the use of: an overlay dielectric medium, composite substrate of different materials, suspended substrate, splitted conductors, a parallel slot or a tuning septum in the ground plane (see, for example, K. Sachse, A. Sawicki, Quasi-Ideal Multilayer Two- and Three-Strip Directional Couplers for Monolithic and Hybrid MICs, IEEE Transactions on Microwave Theory and Techniques, vol. 47, No. 9, September 1999, pp. 1873–1882). Uniaxial dielectric materials, wiggled coupled lines, and external compensation with lumped capacitors is also used—the latest one allows only narrow frequency band compensation. Some of the mentioned above techniques are suitable for weakly coupled lines, some of them—for tightly coupled lines. In multilayer topologies vertical connections to the input and output lines (see for example U.S. Pat. No. 6,208,220 B1, and JP63043402 patents), or between the multiple coupled lines (see for example U.S. Pat. No. 5,629,654 patent) are provided utilizing via-holes. Vertical interconnections can be easily applied in printed circuit board (PCB), low temperature cofired ceramic (LTCC), and microwave monolithic integrated circuits (MMIC) technologies.
The known configurations of coupled-lines structures manufacured in PCB or LTCC technologies are not compensated. In most common cases a final board is built of a few layers of substrates with the same dielectric permittivity. The compensation technique of using dielectric substrates with different dielectric permittivities can be seldom applied. Weakly coupled lines can be compensated using lumped capacitors mounted on the top layer, or tooth- or comb-type shape of coupled lines can be used. Unfortunately, these techniques are very sensitive on dimensions tolerances of the printed lines, and on tolerances of parameters of the applied components. There is not any known technique to compensate tightly-coupled lines manufactured in the classical PCB or LTCC technology, where the same dielectric material is used to build a multilayer coupled-lines structure. The use of different dielectric materials results in a more complicated manufacturing process, and therefore relatively high costs. Additionally, different dielectric materials have different coefficients of thermal expansion. The difference of said coefficients will cause a temperature change to induce stresses in the substrates. It is difficult to find dielectric substrates with similar thermal coefficients and the required values of dielectric permittivity at the same time. Moreover, to bond different substrate materials a thermoplastic or a thermoset film must be used, which is adapted to bond the two specific materials together. Such films are difficult, if not impossible to obtain.